A man breathes through an oxygen mask as another one receives treatments, after a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017. REUTERS/Ammar Abdullah

A Britain-based monitoring group claimed at least 58 people, including 11 children, had been killed in the town of Khan Sheikhun in Idlib province by a gas attack. Warplanes carried out a suspected toxic gas attack in the rebel-held town of northwestern Syria on Tuesday. The Britain-based monitoring group was unable to confirm the nature of the substance used in the attack. It was unclear if the planes involved were Syrian.

According to the Syrian Observatory for Human Rights, “Those killed in the town of Khan Sheikhun, in Idlib province, had died from the effects of the gas, adding that dozens more suffered respiratory problems and other symptoms.” Syria’s opposition National Coalition has accused President Bashar al-Assad’s government of the attack and demanded a UN investigation.

The gas attack happened before the start of a two-day conference on Syria’s future in Brussels by the European Union. Idlib province is mostly controlled by an alliance of rebels including former Al-Qaeda affiliate Fateh al-Sham Front, which fights the Assad regime.

More than 3,00,000 people have been killed in Syria since the conflict began in March 2011.